Synchronous drive systems, such as timing belt based systems, are widely used in motor vehicles, as well as in industrial applications. In motor vehicles, for example, timing belts or chains are used to drive the camshafts that open and close the engine intake and exhaust valves. Also other devices such as water pumps, fuel pumps etc. can be driven by the same belt or chain.
Internal combustion engines produce many types of mechanical vibrations during their operation, and these vibrations are usually transmitted through the timing belt or chain in the synchronous drive system. A particularly intense source of mechanical vibrations is given by the intake and exhaust valves and the camshafts that open and close those intake and exhaust valves. Opening and closing the intake and exhaust valves leads to a type of vibration known as torsional vibration. When the frequency of these vibrations is close to the natural frequency of the drive, system resonance occurs. In resonance the torsional vibrations and the span tension fluctuations are at their maximum.
Torsional vibrations cause fluctuations in belt or chain tension, which can lead to increased wear and reduced belt or chain life. Torsional vibrations may also cause timing errors, and result in undesirable amounts of noise.
It is known to provide non-circular sprocket components in such drive systems to attempt to reduce or eliminate vibration. In DE-A-195 20 508 (Audi AG), there is disclosed a wrapped belt drive for an internal combustion engine, the timing belt being wrapped around two driven pulleys coupled to the camshaft of the engine, and one drive pulley coupled to the crankshaft of the engine. It is proposed in the citation to reduce torsional vibrations by an “out of round” pulley, which is shown as consisting of one of the camshaft pulleys. The out of round pulley which is shown has four protruding portions and four receding portions arranged regularly around the pulley.
In Japanese Utility Model JP 62-192077 (Patent Bulletin No. HEI 1-95538) of 1987 (Hatano et al/Mitsubishi), there is disclosed a tension equalising driving device which transmits the rotation of a drive pulley to a driven pulley by a belt drive such as a timing belt in an internal combustion engine. There is shown a timing belt arrangement in which a toothed pulley of the drive shaft of a camshaft is driven by an oval timing belt driving sprocket connected to the drive shaft of an internal combustion engine. In Japanese Application No. HEI 9-73581 (Patent Bulletin No. HEI 10-266868) of 1997 (Kubo/Mitsubishi) reference is also made to the use of an oval sprocket as a crank sprocket.